net/http/httputil: make https DumpRequestOut less racy
net/http/httputil: make https DumpRequestOut less racy

It's still racy in that it mutates req.Body, though.  *shrug*

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5709054

wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old